The existing chilled water plant is a standard primary-secondary configuration with constant flow pumping in the primary loop <and secondary loop>. The cooling tower fans come on in stages to maintain a fixed condenser water temperature. The chillers stage on and compressor vanes modulate to maintain a fixed chilled water setpoint.
This type of operation is the most straightforward to control, but does not account for varying load conditions and component interactions to maximize operating efficiency.
Chilled water plants are complex systems in which component s interact in ways that are not always obvious. There are many ways to configure system piping, pumping, and controls to deliver the same cooling result, but there is no single approach that is best for all situations.
With increased use of digital control systems in recent years, there has been a lot of development work in understanding component interactions and applying a systems approach to optimizing plants for energy efficiency. At the same time, the price of variable frequency motor drives has been dropping and the technology has become much more reliable. The HVAC industry has come to recognize that best overall plant efficiency can be achieved with broad use of VFD’s for chilled water pumping, condenser water pumping, cooling tower fans, and centrifugal chiller compressors, in combination with a purpose-built intelligent control system to manage the added complexity.
The capital cost of new variable speed chillers are usually not justified on energy savings, so we are recommending installation of VFD’s on the pumps and tower fans, along with new inverter-duty motors. The control system would be replaced with an integrated purpose-built system to optimize the plant as a whole.
Issues and Concerns
McQuay publishes that adding a VFD to a single-compressor chiller will save 0.14 kW/ton IPLV (.505 - .365). Armstrong claims that ultra-efficient Hartman Loop controls with an all variable speed plant will achieve <0.45 kW/ton compared to a typical plant of 0.75 for a good new plant to 1.1 kW/ton for an older plant. A standard code-based plant will be 0.75 to 0.9 kW/ton. If the chiller is not retrofitted, deduct that portion of the savings. All numbers include condenser pumps and tower fans, but not chilled water pumps.
Add allowance for control panel and external control upgrade. Contact chiller manufacturer.